Steel sheet and method for manufacturing same

ABSTRACT

A steel sheet has a predetermined chemical composition, in which a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 0.01% to 5.0%, a metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 2.0% to 10.0%, the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, and in the surface layer region, an average grain size of the secondary phase is 0.01 μm to 4.0 μm, and a texture in which an X ODF{001}/{111}  as the ratio of the intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.60 or more and less than 2.00 is included.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a steel sheet and a method formanufacturing the same.

Priority is claimed on Japanese Patent Application No. 2019-000672,filed on Jan. 7, 2019, the content of which is incorporated herein byreference.

RELATED ART

Recently, in order to protect the global environment, it is desired toimprove the fuel consumption of a vehicle. Regarding the improvement ofthe fuel consumption of a vehicle, high-strengthening is furtherrequired for a steel sheet for a vehicle in order to reduce the weightof a vehicle body while securing safety. This high-strengthening isrequired not only for a structural member such as a member or a pillarbut also for an exterior component (for example, a roof, a hood, afender, or a door) of a vehicle. For this requirement, a material hasbeen developed in order to simultaneously achieve strength andelongation (formability).

On the other hand, the forming of an exterior component of a vehicletends to become more complicated. When the thickness of a steel sheet isreduced through high-strengthening, a surface of the steel sheet islikely to be uneven during forming into a complicated shape. When thesurface is uneven, the external appearance after forming deteriorates.Regarding an exterior panel component, not only characteristics such asstrength but also design and surface quality are important. Therefore,the external appearance after forming is required to be excellent. Theunevenness occurring after forming described herein refers to unevennessoccurring on a surface of a formed component even when the steel sheetsurface after manufacturing is not uneven. Even when the formability ofthe steel sheet is improved, the occurrence is not necessarilysuppressed. Therefore, when a high strength steel sheet is applied to anexterior panel of a vehicle, there is a large problem.

Regarding a relationship between the external appearance after formingand material characteristics in a steel sheet to be applied to anexterior panel, for example, Patent Document 1 discloses a ferriticsteel sheet in which, in order to improve surface properties afterstretching, an area fraction of crystal having a crystal orientation of±15° from {001} plane parallel to a steel sheet surface is 0.25 or lessand an average grain size of the crystal is 25 μm or less.

However, Patent Document 1 relates to a ferritic steel sheet in which aC content is 0.0060% or less. For high-strengthening of a steel sheet,it is effective to increase the C content such that a dual phasestructure including ferrite and a hard phase is obtained. However, as aresult of an investigation by the present inventors, it was found that,when the C content is increased to obtain a dual phase structure, thearea fraction of crystal having a crystal orientation of ±15° from {001}plane parallel to a steel sheet surface cannot be reduced unlike PatentDocument 1. That is, with the method disclosed in Patent Document 1, thehigh-strengthening and the improvement of surface properties afterworking (suppression of the occurrence of unevenness) cannot be achievedsimultaneously.

For example, Patent Document 2 discloses a dual phase structure steelincluding ferrite and a secondary phase, and states that it is effectiveto decrease the yield point as a countermeasure against surface strainduring forming. However, Patent Document 2 does not disclose arelationship between the external appearance after forming and astructure from the viewpoint of a countermeasure against surfaceroughness or pattern.

That is, in the related art, a high-strength dual phase structure steelin which surface roughness or pattern defects after forming is improvedis not disclosed.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2016-156079

[Patent Document 2] PCT International Publication No. WO2013/046476

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in consideration of theabove-described problems. An object of the present invention is toprovide: a high strength steel sheet in which the occurrence of surfaceunevenness during forming is suppressed; and a method for manufacturingthe same.

Means for Solving the Problem

The present inventors conducted an investigation on a method forachieving the object. In particular, the present inventors conducted athorough investigation on a relationship between surface unevenness in amanufactured steel sheet or surface unevenness after forming and amicrostructure or a texture of the steel sheet and found that: i)unevenness after forming occurs even when unevenness does not occur onthe steel sheet surface after manufacturing; ii) surface unevennessafter forming occurs due to inhomogeneity of deformation in a range fromthe steel sheet surface to a position of 20 μm in the sheet thicknessdirection; and iii) the reason for the uneven deformation is non-uniformdispersion of a hard structure or development of a specific texture.

In addition, as a result of further investigation, the present inventorsfound that DP (dual phase) steel including ferrite and a secondary phaseis preferable in order to simultaneously achieve strength andformability, and by adjusting the fraction of the secondary phase, theaverage grain size of the secondary phase, and the texture of ferrite inthe metallographic structure in a surface layer region ranging from thesurface to a range of 0 to 20 μm in the sheet thickness direction to bedifferent from those in an internal region of the steel sheet, a steelsheet in which the occurrence of surface unevenness after forming issuppressed and the external appearance (surface appearance quality)after forming is excellent can be obtained, while securing strength.

In addition, as a result of investigation, the present inventors foundthat, in order to control the metallographic structure in the surfacelayer region, it is effective to apply strain after hot-rolling insteadof after cold-rolling and to set a cold-rolling reduction and heattreatment conditions after the strain application depending on theworking amount.

The present invention has been made based on the above findings, and thescope thereof is as follows.

(1) According to one aspect of the present invention, there is provideda steel sheet including, as a chemical composition, by mass %: C: 0.020%to 0.090%; Si: 0.200% or less; Mn: 0.45% to 2.10%; P: 0.030% or less; S:0.020% or less; sol. Al: 0.50% or less; N: 0.0100% or less; B: 0% to0.0050%; Mo: 0% to 0.40%; Ti: 0% to 0.10%; Nb: 0% to 0.10%; Cr: 0% to0.55%; Ni: 0% to 0.25%; and a remainder of Fe and impurities, in which ametallographic structure in a surface layer region ranging from asurface to a position of 20 μm from the surface in a sheet thicknessdirection consists of ferrite and a secondary phase having a volumefraction of 0.01% to 5.0%, a metallographic structure in an internalregion ranging from a position of more than 20 μm from the surface inthe sheet thickness direction to a ¼ thickness position from the surfacein the sheet thickness direction consists of ferrite and a secondaryphase having a volume fraction of 2.0% to 10.0%, the volume fraction ofthe secondary phase in the surface layer region is less than the volumefraction of the secondary phase in the internal region, and in thesurface layer region, the average grain size of the secondary phase is0.01 μm to 4.0 μm, and the texture in which an X_(ODF{001}/{111}) in asthe ratio of an intensity of {001} orientation to an intensity of {111}orientation in the ferrite is 0.60 or more and less than 2.00 isincluded.

(2) In the steel sheet according to (1), the average grain size of thesecondary phase in the internal region may be 1.0 μm to 5.0 μm and maybe more than the average grain size of the secondary phase in thesurface layer region.

(3) In the steel sheet according to (1) or (2), in a range of 0μm to 50μm from the surface in the sheet thickness direction in a cross sectionof the steel sheet orthogonal to a rolling direction, the average numberdensity of the secondary phase per observed viewing field having alength of 100 μm in a sheet width direction and a length of 50 μm in thesheet thickness direction may be 130 or less, and the minimum numberdensity of the secondary phase per the observed viewing field may bemore than or equal to a value obtained by subtracting 20 from theaverage number density of the secondary phase.

(4) In the steel sheet according to any one of (1) to (3), the chemicalcomposition may include, by mass %, one or more selected from the groupconsisting of: B: 0.0001% to 0.0050%; Mo: 0.001% to 0.40%; Ti: 0.001% to0.10%; Nb: 0.001% to 0.10%; Cr: 0.001% to 0.55%; and Ni: 0.001% to0.25%.

(5) In the steel sheet according to any one of (1) to (4), the secondaryphase in the surface layer region may include one or more selected fromthe group consisting of martensite, bainite, and tempered martensite.

(6) In the steel sheet according to any one of (1) to (5), a platinglayer may be provided on the surface.

(7) In the steel sheet according to any one of (1) to (6), a tensilestrength may be 400 MPa or higher.

(8) According to another aspect of the present invention, there isprovided a method for manufacturing a steel sheet including: a heatingprocess of heating a slab having the chemical composition according to(1) at 1000° C. or higher; a hot-rolling process of hot-rolling the slabsuch that a rolling finishing temperature is 950° C. or lower to obtaina hot-rolled steel sheet; a stress application process of applying astress to the hot-rolled steel sheet after the hot-rolling process suchthat an absolute value of a residual stress σs on a surface is 150 MPato 350 MPa; a cold-rolling process of cold-rolling the hot-rolled steelsheet after the stress application process such that a cumulativerolling reduction R_(CR) is 70% to 90% to obtain a cold-rolled steelsheet; an annealing process of heating the cold-rolled steel sheet suchthat an average heating rate in a range from 300° C. to a soakingtemperature T1° C. that satisfies the following Expression (i) is 1.5°C./sec to 10.0° C./sec and holding the heated steel sheet at the soakingtemperature T1° C. for 30 seconds to 150 seconds for annealing; and acooling process of cooling the cold-rolled steel sheet after theannealing process to 550° C. to 650° C. such that an average coolingrate in a range from T1° C. to 650° C. is 1.0° C./sec to 10.0° C./secand cooling the cooled steel sheet to 200° C. to 490° C. such that theaverage cooling rate is 5.0° C./sec to 500.0° C./sec.

1275−27−1n(σs)−4.5×R _(CR) ≤T1≤1275−27×1n(σs)−6×R _(CR)   (i)

(9) In the method for manufacturing a steel sheet according to (8), thestress application process may be performed at 40° C. to 500° C.

(10) In the method for manufacturing a steel sheet according to (8) or(9), in the hot-rolling process, a finish rolling start temperature maybe 900° C. or lower.

(11) The method for manufacturing a steel sheet according to any one of(8) to (10) may further include a holding process of holding thecold-rolled steel sheet after the cooling process in a temperature rangeof 200° C. to 490° C. for 30 seconds to 600 seconds.

Effects of the Invention

In the steel sheet according to the aspect of the present invention, theoccurrence of surface unevenness is suppressed even after variousdeformation during press forming as compared to a material in therelated art. Therefore, the steel sheet according to the aspect of thepresent invention has excellent appearance quality of the surface andcan contribute to improvement of the vividness and design of coating. Inaddition, the steel sheet according to the present invention has highstrength and can contribute to further reduction in the weight of avehicle. In the present invention, the high strength represents atensile strength of 400 MPa or higher.

In addition, with the method for manufacturing a steel sheet accordingto the aspect of the present invention, a high strength steel sheet inwhich the occurrence of surface unevenness is suppressed even aftervarious deformation during press forming can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the effect of a texture parameter on surfaceappearance quality after forming, in which a solid square symbol of theplot represents an example of a range in which a fraction of a secondaryphase in a surface layer is not preferable.

EMBODIMENTS OF THE INVENTION

A steel sheet according to an embodiment of the present invention (thesteel sheet according to the embodiment) includes, as a chemicalcomposition, by mass %: C: 0.020% to 0.090%; Si: 0.200% or less; Mn:0.45% to 2.10%; P: 0.030% or less; S: 0.020% or less; sol. Al: 0.50% orless; and N: 0.0100% or less, and optionally further includes: B:0.0050% or less; Mo: 0.40% or less; Ti: 0.10% or less; Nb: 0.10% orless; Cr: 0.55% or less; Ni: 0.25% or less; and a remainder of Fe andimpurities.

In addition, in the steel sheet according to the embodiment, ametallographic structure in a surface layer region ranging from asurface to a position of 20 μm from the surface in a sheet thicknessdirection consists of ferrite and a secondary phase having a volumefraction of 0.01% to 5.0%, a metallographic structure in an internalregion ranging from a position of more than 20 μm from the surface inthe sheet thickness direction to a ¼ thickness position from the surfacein the sheet thickness direction consists of ferrite and a secondaryphase having a volume fraction of 2.0% to 10.0%, and the volume fractionof the secondary phase in the surface layer region is less than thevolume fraction of the secondary phase in the internal region.

In addition, in the steel sheet according to the embodiment, in thesurface layer region, an average grain size of the secondary phase is0.01 μm to 4.0 μm, and a texture in which anX_(ODF{001}/{111} as the ratio of the intensity of {)001} orientation toan intensity of {111} orientation in the ferrite is 0.60 or more andless than 2.00 is included.

In the steel sheet according to the embodiment, it is preferable that anaverage grain size of the secondary phase in the internal region is 1.0μm to 5.0 μm and is more than the average grain size of the secondaryphase in the surface layer region. In addition, it is preferable that,in a range of 0μm to 50 μm from the surface in the sheet thicknessdirection in a cross section of the steel sheet orthogonal to a rollingdirection, an average number density of the secondary phase per observedviewing field having a length of 100 μm in a sheet width direction and alength of 50 μm in the sheet thickness direction is 130 or less, and aminimum number density of the secondary phase per observed viewing fieldhaving a size of 100 μm×50 μm is more than or equal to a value obtainedby subtracting 20 from the average number density of the secondaryphase.

Hereinafter, the steel sheet according to the embodiment will bedescribed in detail.

<Regarding Chemical Composition>

First, the reason for limiting the chemical composition of the steelsheet according to the embodiment will be described.

[C: 0.020% to 0.090%]

C (carbon) is an element that increases the strength of the steel sheetand is essential for securing the volume fraction of the secondaryphase. In order to secure a predetermined volume fraction of thesecondary phase, the C content is set to be 0.020% or more. The Ccontent is preferably 0.030% or more.

On the other hand, when the C content is more than 0.090%, the number ofhard phase (secondary phase) grains increases, and the hard phase islikely to be linked. A portion other than the linked hard phase ispromoted to be deformed during forming. In a case where hard phasegrains are non-uniformly dispersed, pattern defects are likely to beformed on the surface after forming. In addition, when the C content ismore than 0.090%, a cold-rolling force during cold-rolling at a highrolling reduction increases, the productivity decreases, and theformability or weldability of the steel sheet deteriorates. Therefore,the C content is set to be 0.090% or less. The C content is preferably0.070% or less and more preferably 0.060% or less.

[Si: 0.200% or less]

Si (silicon) is a deoxidizing element of steel that is effective forincreasing the mechanical strength of the steel sheet. However, when theSi content is more than 0.200%, scale peelability during productiondeteriorates, and surface defects are likely to be formed on theproduct. In addition, a cold-rolling force during cold-rolling at a highrolling reduction increases, and the productivity decreases. Further,the weldability or the deformability of the steel sheet deteriorates.Therefore, the Si content is set to be 0.200% or less. The Si content ispreferably 0.150% or less and more preferably 0.100% or less.

[Mn: 0.45% to 2.10%]

Mn (manganese) is an element that is effective for increasing themechanical strength of the steel sheet. In addition, Mn is an elementthat immobilizes S (sulfur) in the steel as MnS or the like to preventcracking during hot-rolling. In order to obtain the effects, the Mncontent is set to be 0.45% or more.

On the other hand, when the Mn content is more than 2.10%, acold-rolling force during cold-rolling at a high rolling reductionincreases, and the productivity decreases. In addition, segregation ofMn is likely to occur. Therefore, the hard phase aggregates afterannealing such that pattern defects are likely to be formed on thesurface after forming. Therefore, the Mn content is set to be 2.10% orless. The Mn content is preferably 2.00% or lower.

[P: 0.030% or less]

P (phosphorus) is an impurity. When an excess amount of P is included inthe steel, cracking is promoted during hot-rolling or cold-rolling, andthe weldability or ductility of the steel sheet deteriorates. Therefore,the P content is limited to 0.030% or less. It is preferable that the Pcontent is limited to 0.020% or less. The P content is preferably smalland may be 0%. In consideration of existing general refining (includingsecondary refining), the P content may be 0.0005% or more.

[S: 0.020% or less]

S (sulfur) is an impurity. When an excess amount of S is included in thesteel, MnS stretched by hot-rolling is formed, and the deformability ofthe steel sheet deteriorates. Therefore, the S content is limited to0.020% or less. The S content is preferably small and may be 0%. Inconsideration of existing general refining (including secondaryrefining), the S content may be 0.0005% or more.

[sol. Al: 0.50% or less]

Al (aluminum) is a deoxidizing element of steel that is effective forincreasing the mechanical strength of the steel sheet. In order toobtain the effect, the sol. Al content may be 0.10% or more. In a casewhere deoxidation is performed using Si, sol. Al does not need to beincluded.

On the other hand, when the sol. Al content is more than 0.50%, thecastability deteriorates, and the productivity deteriorates. Therefore,the sol. Al content is set to be 0.50% or less.

[N: 0.0100% or less]

N (nitrogen) is an impurity and is an element that deteriorates thedeformability of the steel sheet. Accordingly, the N content is limitedto 0.0100% or less. The N content is preferably small and may be 0%.However, in consideration of existing general refining (includingsecondary refining), the N content may be 0.0005% or more.

That is, the steel sheet according to the embodiment may include theabove-described elements and a remainder consisting of Fe andimpurities. However, in order to improve various characteristics, thefollowing elements (optional elements) may be included instead of a partof Fe. From the viewpoint of reducing the alloy cost, it is notnecessary to add the optional elements to the steel on purpose.Therefore, the lower limit of the amount of each of the optionalelements is 0%.

The impurities refer to components that are unintentionally includedfrom raw materials or other manufacturing processes in the process ofmanufacturing the steel sheet.

[B: 0% to 0.0050%]

B (boron) is an element that immobilizes carbon and nitrogen in thesteel to form a fine carbonitride. The fine carbonitride contributes toprecipitation hardening, microstructure control, grain refinementstrengthening, and the like of the steel. Therefore, B may be optionallyincluded. In order to obtain the effect, the B content is preferably0.0001% or more.

On the other hand, when the B content is more than 0.0050%, the effectis saturated, and the workability (deformability) of the steel sheet maydeteriorate. Therefore, even in a case where B is included, the Bcontent is set to be 0.0050% or less. In addition, the strength of thecold-rolled original sheet increases by including B. Therefore, acold-rolling force during cold-rolling at a high rolling reductionincreases. Therefore, from the viewpoint of productivity, the B contentis preferably 0.0005% or less.

[Mo: 0% to 0.40%]

Mo (molybdenum) is an element that contributes to the improvement of themechanical strength of the steel sheet. In addition, in a case where theMo content is less than the Mn content, Mo is an element that is lesslikely to segregate than Mn and contributes to uniform dispersion of thehard phase. Therefore, Mo may be optionally included. In order to obtainthe effect, the Mo content is preferably 0.001% or more.

On the other hand, when the Mo content is excessively large, thedeformability of the steel sheet may deteriorate. Therefore, even in acase where Mo is included, the Mo content is 0.40% or less. In addition,Mo is an expensive element, and an increase in Mo content increases anincrease in alloy cost. From this viewpoint, the Mo content ispreferably 0.15% or less.

[Ti: 0% to 0.10%]

Ti (titanium) is an element that immobilizes carbon and nitrogen in thesteel to form a fine carbonitride. The fine carbonitride contributes toprecipitation hardening, microstructure control, grain refinementstrengthening, and the like of the steel. Therefore, Ti may beoptionally included. In order to obtain this effect, the Ti content ispreferably 0.001% or higher.

On the other hand, when the Ti content is more than 0.10%, the effect issaturated, the strength of the cold-rolled original sheet (steel sheetprovided for cold-rolling) increases, and a cold-rolling force duringcold-rolling at a high rolling reduction increases. Therefore, even in acase where Ti is included, the Ti content is 0.10% or less.

[Nb: 0% to 0.10%]

Nb (niobium) is an element that immobilizes carbon and nitrogen in thesteel to form a fine carbonitride. The fine Nb carbonitride contributesto precipitation hardening, microstructure control, grain refinementstrengthening, and the like of the steel. Therefore, Nb may beoptionally included. In order to obtain the effect, the Nb content ispreferably 0.001% or more.

On the other hand, when the Nb content is more than 0.10%, the effect issaturated, the strength of the cold-rolled original sheet increases, anda cold-rolling force during cold-rolling at a high rolling reductionincreases. Therefore, even in a case where Nb is included, the Nbcontent is 0.10% or less.

[Cr: 0% to 0.55%]

Cr (chromium) is an element that contributes to the improvement of themechanical strength of the steel sheet. Therefore, Cr may be optionallyincluded. In order to obtain the effect, the Cr content is preferably0.001% or more.

On the other hand, when the Cr content is excessively large, thestrength of the cold-rolled original sheet increases, and a cold-rollingforce during cold-rolling at a high rolling reduction increases. Inaddition, excessive inclusion of Cr causes an increase in alloy cost.Therefore, even in a case where Cr is included, the Cr content is 0.55%or less.

[Ni: 0% to 0.25%]

Ni (nickel) is an element that contributes to the improvement of themechanical strength of the steel sheet. Therefore, Ni may be optionallyincluded. In order to obtain the effect, the Ni content is preferably0.001% or more.

On the other hand, when the Ni content is excessively large, thestrength of the cold-rolled original sheet increases, and a cold-rollingforce during cold-rolling at a high rolling reduction increases. Inaddition, excessive inclusion of Ni causes an increase in alloy cost.Therefore, even in a case where Ni is included, the Ni content is 0.25%or less.

<Metallographic Structure of Surface Layer Region>

In the steel sheet according to the embodiment, when the sheet thicknessis represented by t, the depth range from the surface to t/4 in a sheetthickness direction is divided into two regions, a depth range from thesurface as a starting point to a depth position of 20 μm in a depthdirection as an end point is represented by a surface layer region, anda range over the surface layer region to a center side of the steelsheet is represented by an internal region.

As a result of a thorough investigation by the present inventors, it wasfound that the surface unevenness during forming occurs due toinhomogeneous deformation occurs during forming caused by inhomogeneityin strength in a microscopic region. In particular, it was found thatthe occurrence of the unevenness of the surface is largely affected bythe metallographic structure in the surface layer region ranging fromthe surface to a range of 0 to 20 μm in the sheet thickness direction(range from the surface to a position of 20 μm in the sheet thicknessdirection). Therefore, in the steel sheet according to the embodiment,the metallographic structure in the surface layer region is controlledas follows.

[Consisting of Ferrite and Secondary phase having Volume Fraction of0.01% to 5.0% and Volume fraction of Secondary Phase being less thanVolume fraction of Secondary Phase in Internal region]

When the volume fraction of the secondary phase in the surface layerregion is less than 0.01%, the strength of the steel sheet is notsufficiently improved. Therefore, the volume fraction of the secondaryphase is set to be 0.01% or more.

On the other hand, when the volume fraction of the secondary phase ismore than 5.0%, the hard phase is likely to be non-uniformly dispersed.Therefore, surface unevenness occurs during forming, and the externalappearance after forming deteriorates.

In addition, the volume fraction of the secondary phase in themetallographic structure of the surface layer region is set to be lessthan the volume fraction of the secondary phase in the internal region.By setting the volume fraction of the secondary phase in the surfacelayer to be less than the volume fraction of the secondary phase in theinternal region and further increasing the volume fraction in theinternal region, the suppression of the occurrence of the surfaceunevenness and the material strength can be achieved simultaneously.

In the steel sheet according to the embodiment, the secondary phase is ahard structure other than ferrite and is, for example, one or more amongpearlite, martensite, residual austenite, bainite, and temperedmartensite. From the viewpoint of improving strength, one or more amongmartensite, bainite, and tempered martensite is preferable, andmartensite is more preferable.

The volume fraction of the secondary phase in the surface layer regioncan be obtained using the following method.

A sample (the size is substantially 20 mm in the rolling direction×20 mmin the width direction x the thickness of the steel sheet) formetallographic structure (microstructure) observation is collected froma W/4 position or a 3W/4 position of a sheet width W of the obtainedsteel sheet (that is, an end portion of the steel sheet in the widthdirection to the W/4 position in the width direction), and ametallographic structure (microstructure) in a range from the surfacelayer to the ¼ thickness position is observed using an opticalmicroscope to calculate the area fraction of the secondary phase in arange from the surface of the steel sheet (in a case where a platinglayer is present, the surface excluding the plating layer) to 20 μm. Inorder to prepare the sample, a sheet thickness cross section in adirection orthogonal to the rolling direction is polished as anobservation section and is etched with the LePera reagent.

“Microstructures” are classified based on an optical microscope image ata magnification by 500-times obtained after etching with the LePerareagent. When the optical microscope observation is performed after theLePera corrosion, the respective structures are observed with differentcolors, for example, bainite is observed to be black, martensite(including tempered martensite) is observed to be white, and ferrite isobserved to be gray. Therefore, ferrite and other hard structures can beeasily distinguished from each other. In the structure image, a regionother than gray representing ferrite is the secondary phase.

A region ranging from the surface layer to a ¼ thickness position in thesteel sheet etched with the LePera reagent is observed in 10 viewingfields at a magnification by 500-times, a region from the surface layerto a position of 20 μm of the steel sheet in the structure image isdesignated, and the image analysis is performed using image analysissoftware “Photoshop CS5” (manufactured by Adobe Inc.) to obtain the areafraction of the secondary phase. In an image analysis method, forexample, a maximum luminosity value Lmax and a minimum luminosity valueLmin of the image are acquired from the image, a portion that has pixelshaving a luminosity of Lmax−0.3×(Lmax−Lmin) to Lmax is set as a whiteregion, a portion that has pixels having a luminosity of Lmin toLmin+0.3×(Lmax−Lmin) is set as a black region, a portion other than thewhite and black regions is set as a gray region, and the area fractionof the secondary phase that is the region other than gray is calculated.By performing the image analysis as described above in 10 viewing fieldsin total, the area fraction of the secondary phase is measured. Assumingthat the area fraction is the same as the volume fraction, the averageof the area fraction values was calculated as the volume fraction of thesecondary phase in the surface layer region.

[Average Grain Size of Secondary Phase being 0.01 to 4.0 μm]

When the average grain size of the secondary phase is more than 4.0 μm,the external appearance after forming deteriorates. Therefore, theaverage grain size of the secondary phase in the surface layer region isset to be 4.0 μm or less.

On the other hand, when the average grain size of the secondary phase isless than 0.01 μm, grains of the secondary phase are likely toaggregate. Even in a case where individual grains of the secondary phaseare fine, when the grains aggregate, the external appearance afterforming deteriorates. Therefore, the average grain size of the secondaryphase is set to be 0.01 μm or more. The average grain size is preferably0.10 μm or more.

The average grain size of the secondary phase in the surface layerregion can be obtained using the following method.

Using the same method as described above, a region ranging from thesurface layer to a ¼ thickness position in the steel sheet etched withthe LePera reagent is observed in 10 viewing fields at a magnificationby 500-times, a region from the surface layer to a position of 20 μm x200 μm of the steel sheet in the structure image is selected, and theimage analysis is performed using image analysis software “PhotoshopCS5” (manufactured by Adobe Inc.) to calculate the area of the secondaryphase and the number of grains of the secondary phase, respectively. Byadding up the values and dividing the area of the secondary phase by thenumber grains of the secondary phase, the average area per grain of thesecondary phase is calculated. The circle equivalent diameter iscalculated based on the area and the number of grains to obtain theaverage grain size of the secondary phase.

[Texture in which X_(ODF{001}/{111}) as Ratio of Intensity of {001}Orientation to Intensity of {111} Orientation in Ferrite is 0.60 or moreand less than 2.00 being included in Surface Layer Region]

When a texture in which an X_(ODF{001}/{111}) as the ratio of theintensity of {001} orientation to an intensity of {111} orientation inthe ferrite (ratio between maximum values of X-ray random intensityratios) is 0.60 or more and less than 2.00 is included in the surfacelayer region, the external appearance after forming is improved. Thereason for this is not clear but is presumed to be that theinhomogeneous deformation on the surface is suppressed due to aninteraction between the existence form of the secondary phase and thecrystal orientation distribution of ferrite.

When X_(ODF{001}/{111}) is less than 0.60, inhomogeneous deformationcaused by an orientation distribution and a difference in intensity ofeach the crystal of the material (steel sheet) is likely to occur, anddeformation concentration on the orientation in the vicinity of {001} inferrite is significant.

On the other hand, it is presumed that, when X_(ODF{001}/{111}) is 2.00or more, inhomogeneous deformation caused by an orientation distributionand a difference in intensity of each the crystal of the material (steelsheet) is likely to occur, inhomogeneous deformation is likely to occurin a boundary between ferrite and the secondary phase and a boundarybetween crystal grains in an orientation in the vicinity of {111} andcrystal grains in another orientation in ferrite, and surface unevennessis likely to occur.

In addition, it is more preferable that a difference betweenX_(ODF{001}/{111}) of ferrite in the surface layer region andX_(ODF{001}/{111}) of ferrite in the internal region is −0.40 to 0.40because inhomogeneous deformation in the ferrite in the sheet thicknessdirection is suppressed, and contributes to the improvement of strainhardening property of the material.

Whether or not the texture in which X_(ODF{001}/{111}) is 0.60 to 2.00is included in ferrite of the surface layer region can be determined inthe following manner using EBSD (Electron Backscattering Diffraction)method.

Regarding a sample provided for EBSD method, the steel sheet is polishedby mechanical grinding, strain is removed by chemical polishing orelectrolytic polishing, the sample is prepared such that the crosssection in the sheet thickness direction including the range from thesurface to the ¼ thickness position is a measurement surface, and thetexture is measured. Regarding a sample collection position in the sheetwidth direction, it is desirable to collect the sample in the vicinityof a sheet width position of W/4 or 3W/4 (position at a distance of ¼from an end surface of the steel sheet in the sheet width direction).

In the region of the sample ranging from the surface of the steel sheetto 20 μm in the sheet thickness direction, a crystal orientationdistribution is measured by EBSD method at a pitch of 0.5 μm or less.Ferrite is extracted using an IQ (Image Quality) map that is analyzableby EBSP-OIM (registered trade name, Electron Backscatter DiffractionPattern-Orientation Image Microscopy). Ferrite has a characteristic inthat the IQ value is high, and thus can be simply classified using thismethod. The threshold of the IQ value is set such that the area fractionof ferrite that is calculated by the observation of the microstructureobtained by the LePera corrosion matches the area fraction of ferritecalculated based on the IQ value.

In a cross section of ϕ2=45° in a three-dimensional texture (ODF:Orientation Distribution Functions) calculated using crystalorientations of the extracted ferrite, a ratio of a maximum value ofX-ray random intensity ratios of a {001} orientation group to a maximumvalue of X-ray random intensity ratios of a {111} orientation group(γ-fiber) (the maximum value of X-ray random intensity ratios of {001}orientation group/the maximum value of X-ray random intensity ratios of{111} orientation group (γ-fiber)) is obtained as X_(ODF{001}/{111}).The X-ray random intensity ratio is a numerical value obtained bymeasuring the diffraction intensity of a standard sample having nopile-up in a specific orientation and the diffraction intensity of asample material by X-ray diffraction under the same conditions anddividing the obtained diffraction intensity of the sample material bythe diffraction intensity of the standard sample. For example, in a casewhere the steel sheet is rolled at a high rolling reduction of 70% orhigher and annealed, the texture is developed, and the X-ray randomintensity of the {111} orientation group (γ-fiber) increases.

Here, {hk1} represents that, when a sample is collected using theabove-described method, the normal direction of a sheet surface isparallel to <hk1>. Regarding the crystal orientation, typically, anorientation orthogonal to a sheet surface is represented by (hk1) or{hk1}. {hk1} is a generic term for equivalent planes, and (hk1)represents each of crystal planes. That is, in the embodiment, abody-centered cubic structure (bcc structure) is targeted. Therefore,for example, the respective planes (111), (-111), (1-11), (11-1),(-1-11), (-11-1), (1-1-1), and (-1-1-1) are equivalent and cannot bedistinguished from each other. In this case, these orientations arecollectively referred to as “{111} orientation group”. The ODFrepresentation is used for representing other orientations of a crystalstructure having low symmetry. Therefore, in the ODF representation,each of orientations is generally represented by (hk1)[uvw]. However, inthe embodiment, attention is paid to the normal direction orientation{hk1} from which the finding that the normal direction orientation of asheet surface has a large effect on the development of unevenness wasobtained. and (hk1) have the same definition.

In a case where the product is a steel sheet including a plating layer,the surface of the steel sheet excluding the plating layer is defined asan origin of the surface layer region.

<Regarding Metallographic Structure in Internal Region>

In the steel sheet according to the embodiment, it is necessary that, ina state where the metallographic structure in the surface layer regionis controlled as described above, a metallographic structure in aninternal region ranging from a position of more than 20 μm from thesurface in the sheet thickness direction to a ¼ thickness position (in acase where the sheet thickness is represented by t: t/4) from thesurface in the sheet thickness direction is also controlled.

[Consisting of Ferrite and Secondary phase having Volume Fraction of2.0% to 10.0%]

When the volume fraction of the secondary phase in the internal regionis less than 2.0%, the strength of the steel sheet cannot besufficiently improved. Therefore, the volume fraction of the secondaryphase is set to be 2.0% or more.

On the other hand, when the volume fraction of the secondary phase ismore than 10.0%, the volume fraction of ferrite decreases excessively,and workability such as elongation or hole expansibility deteriorates.Therefore, the volume fraction of the secondary phase is set to be 10.0%or less.

[Average Grain Size of Secondary Phase being 1.0 μm to 5.0 μm and beingmore than Average Grain Size of Secondary Phase in Surface Layer Region]

When the average grain size of the secondary phase in the internalregion is 1.0 μm to 5.0 μm and is more than an average grain size of thesecondary phase in the surface layer region, the average grain size ofthe secondary phase in the surface layer region is less than that in theinternal region and inhomogeneous deformation in the surface layerregion is suppressed, which is preferable.

Therefore, the average grain size in the internal region may becontrolled to be in the above-described range.

The volume fraction and the average grain size of the secondary phase inthe internal region can be obtained by using a steel sheet etched withthe LePera reagent, selecting a range from a position of more than 20 μmfrom the surface of the sample in the sheet thickness direction to a ¼thickness position, and analyzing the range with the same method as thatof the surface layer region.

In addition, a texture of ferrite in the internal region can be obtainedby selecting a range from a position of more than 20 μm from the surfaceof the sample in the sheet thickness direction to a ¼ thickness positionby the above-described EBSD method and analyzing the range with the samemethod as that of the surface layer region.

When the thickness of the product is more than 0.4 mm, it is preferablethat the internal region is a range from a position of more than 20 μmfrom the surface in the sheet thickness direction to 100 μm.

<Other Structures>

In a range of 0 μm to 50 μm from the surface in the sheet thicknessdirection in a cross section of the steel sheet orthogonal to therolling direction, when the number density of grains of the secondaryphase and an unevenness in the number density are controlled to be inthe following range, the external appearance after forming is furtherimproved, which is preferable. The uniform dispersibility of the hardphase affects the occurrence of unevenness patterns caused by theoccurrence of inhomogeneous deformation during forming in a wider regionin the sheet thickness direction than the crystal orientation or themicrostructural fraction. Therefore, the uniform dispersibility of thehard phase is controlled in a range of 0 to 50 μm from the surface thatis wider than that in the surface layer region (0 to 20 μm from thesurface) in the sheet thickness direction.

[Average Number Density of Secondary Phase per Observed Viewing Field of100 μm (Sheet Width Direction)×50 μm (Sheet Thickness Direction) being130 or less]

[Minimum Number Density of Secondary Phase per the Observed ViewingField of 100 μm (Sheet Width Direction) x 50 μm (Sheet ThicknessDirection) being more than or equal to Value obtained by subtracting 20from Average Number Density of Secondary Phase]

In a case where the average number density of the secondary phase ismore than 130/viewing field when 10 or more viewing fields having a sizeof 100 μm (sheet width direction)×50 μm (sheet thickness direction) in across section of the steel sheet orthogonal to the rolling direction areobserved, hard grains are likely to be non-uniformly dispersed, and theexternal appearance after forming may deteriorate. In addition, when theminimum number density of the secondary phase is less than a valueobtained by subtracting 20 from the average number density of thesecondary phase (there is an unevenness in number density),inhomogeneous deformation in which deformation concentrates on aposition where a number grains of the hard phase are small, and thus theexternal appearance after forming may deteriorate. Therefore, it ispreferable that the average number density in the observed viewing fieldis 130 or less and the difference between the minimum number density andthe average number density is less than 20 (the minimum number densityis less than or equal to a value obtained by subtracting 20 from theaverage number density).

The number density of the grains of the secondary phase and theunevenness in the number density in the range of 0 to 50 μm from thesurface in the sheet thickness direction can be obtained by using asteel sheet etched with the LePera reagent, designating 10 or moreobserved viewing fields of 100 μm (sheet width direction)×50 μm (sheetthickness direction) in a range from the surface of the sample to aposition of 50 μm from the surface in the sheet thickness direction, andcounting the number of grains of the secondary phase in the observedviewing fields.

The average number density of the secondary phase is the average valueof the numbers of grains of the secondary phase measured in the 10 ormore viewing fields. The minimum number density of the secondary phaseis the minimum number density among the measured numbers of grains, andin a case where the average number density of the secondary phase in theobserved viewing fields (100 μm x 50 μm) is 20 or less, the minimumnumber density may be 0 or more.

<Regarding Plating Layer>

The steel sheet according to the embodiment may include a plating layeron the surface (on the surface of the steel sheet). By including theplating layer on the surface, corrosion resistance is improved, which ispreferable.

A plating to be applied is not particularly limited, and examplesthereof include hot-dip galvanizing, hot-dip galvannealing,electrogalvanizing, Zn—Ni plating (electrogalvanizing), Sn plating,Al-Si plating, electrogalvannealing, hot-dip zinc-aluminum alloyplating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dipzinc-aluminum-magnesium alloy-Si plating, and zinc-Al alloy deposition.

<Regarding Sheet Thickness>

The thickness of the steel sheet according to the embodiment is notparticularly limited. However, in a case where the steel sheet isapplied to an exterior member, when the sheet thickness is more than0.55 mm, the contribution to a reduction in the weight of the member issmall. In addition, when the sheet thickness is less than 0.15 mm, theremay be a problem in rigidity. Therefore, the sheet thickness ispreferably 0.15 mm to 0.55 mm.

<Regarding Manufacturing Method>

Next, a preferable method for manufacturing the steel sheet according tothe embodiment will be described. The effects can be obtained as long asthe steel sheet according to the embodiment has the above-describedcharacteristics irrespective of the manufacturing method. However, withthe following method, the steel sheet can be stably manufactured, whichis preferable.

Specifically, the steel sheet according to the embodiment can bemanufactured with a manufacturing method including the followingprocesses (i) to (vi).

(i) A heating process of heating a slab having the above-describedchemical composition at 1000° C. or higher.

(ii) A hot-rolling process of hot-rolling the slab such that a rollingfinishing temperature is 950° C. or lower to obtain a hot-rolled steelsheet.

(iii) A stress application process of applying a stress to thehot-rolled steel sheet after the hot-rolling process such that anabsolute value of a residual stress ns on a surface is 150 MPa to 350MPa.

(iv) A cold-rolling process of cold-rolling the hot-rolled steel sheetafter the stress application process such that a cumulative rollingreduction R_(CR) is 70% to 90% to obtain a cold-rolled steel sheet.

(v) An annealing process of heating the cold-rolled steel sheet suchthat an average heating rate in a range from 300° C. to a soakingtemperature T1° C. that satisfies the following Expression (1) is 1.5°C./sec to 10.0° C./sec and holding the heated steel sheet at the soakingtemperature T1° C. for 30 seconds to 150 seconds for annealing.

1275−27×1n(σs)−4.5×R _(CR) ≤T1≤1275−27×1n(σs)−4R _(CR)   (1)

(vi) A cooling process of cooling the cold-rolled steel sheet after theannealing process to a temperature range of 550° C. to 650° C. such thatan average cooling rate in a range from T1° C. to 650° C. is 1.0° C./secto 10.0° C./sec and further cooling the cooled steel sheet to atemperature range of 200° C. to 490° C. such that the average coolingrate is 5.0° C./sec to 500.0° C./sec.

In addition, in a case where ductility is improved by temperingmartensite such that a cold-rolled steel sheet or a plated steel sheethaving higher formability is obtained, the manufacturing method mayfurther include the following process.

(vii) A holding process of holding the cold-rolled steel sheet after thecooling process in a temperature range of 200° C. to 490° C. for 30seconds to 600 seconds.

The each process will be described.

[Heating Process]

In the heating process, a slab having the predetermined chemicalcomposition is heated to 1000° C. or higher before rolling. When theheating temperature is lower than 1000° C., a rolling reaction forceduring hot-rolling increases, sufficient hot-rolling cannot beperformed, and there may be a case where the desired thickness of theproduct cannot be obtained. Alternatively, there may a concern that thesteel sheet cannot be coiled due to deterioration in the sheet shape.

It is not necessary to limit the upper limit of the heating temperature,and it is not preferable that the heating temperature is excessivelyhigh from the viewpoint of economy. Due to this reason, it is desirablethat the upper limit of the slab heating temperature is lower than 1300°C.

In addition, the slab provided for the heating process is not limited.For example, a slab that is manufactured using a continuous castingmethod after preparing molten steel having the above-descried chemicalcomposition using an converter or an electric furnace can be used. Forexample, an ingot-making method or a thin slab casting method may beadopted instead of the continuous casting method.

[Hot-Rolling Process]

In the hot-rolling process, the slab heated to 1000° C. or higher in theheating process is hot-rolled and coiled to obtain a hot-rolled steelsheet.

When the rolling finishing temperature is higher than 950° C., theaverage grain size excessively increases. In this case, the averagegrain size of the final product sheet increases, and an increase inaverage grain size causes a decrease in yield strength and deteriorationin the surface appearance quality after forming, which is notpreferable. Therefore, the finish rolling temperature (finish rollingfinishing temperature) is set to be 950° C. or lower. The finish rollingstart temperature is preferably 900° C. or lower.

When a temperature change (finish rolling finishing temperature−finishrolling start temperature) in the hot-rolling process is +5° C. orhigher, recrystallization is promoted by deformation heating in thehot-rolling process, and crystal grains are refined, which ispreferable.

In addition, in order to refine crystal grains, the coiling temperatureis preferably 750° C. or lower and more preferably 650° C. or lower. Inaddition, from the viewpoint of reducing the strength of the cold-rolledoriginal sheet, the coiling temperature is preferably 450° C. or higherand more preferably 500° C. or higher.

[Stress Application Process]

In the stress application process, a stress is applied to the hot-rolledsteel sheet after the hot-rolling process such that an absolute value ofa residual stress as on a surface is 150 MPa to 350 MPa. For example, astress can be applied by grinding the hot-rolled steel sheet using asurface grinding brush after hot-rolling or pickling. At that time,while changing a contact pressure of the grinding brush on the steelsheet surface, a surface layer residual stress is measured on-line usinga portable X-ray residual stress analyzer and may be controlled to be inthe above-described range.

By performing predetermined cold-rolling, annealing, and cooling in astate where the residual stress is applied to the surface to be in theabove-described range, a steel sheet including ferrite having apredetermined texture and having a predetermined hard phase distributioncan be obtained.

When the residual stress is lower than 150 MPa or higher than 350 MPa,the predetermined texture of ferrite cannot be obtained aftercold-rolling, annealing, and cooling to be performed after the stressapplication. In addition, in a case where the residual stress is appliedafter cold-rolling instead of after hot-rolling, the residual stress iswidely distributed in the sheet thickness direction. Therefore, thepredetermined hard phase distribution and the texture cannot be obtainedonly on the surface layer of the material.

A method for applying the residual stress to the surface of thehot-rolled steel sheet is not limited to the above-described grindingbrush. For example, a method for performing shot blasting may also beused. In the case of shot blasting, fine unevenness may occur on thesurface due to collision with shot media, or shot media may be trappedand cause defects during the next cold-rolling or the like. Therefore,the method for applying the stress by grinding using a brush ispreferable.

In addition, during rolling using a roll such as a skin pass, a stressis applied to the entire steel sheet in the thickness direction and thedesired hard phase distribution and the texture cannot be obtained onlyon the surface layer of the material.

It is preferable that the stress application process is performed at asteel sheet temperature of 40° C. to 500° C. By performing the stressapplication process in this temperature range, the residual stress canbe efficiently applied to the range corresponding to the surface layerregion, and the cracking caused by the residual stress of the hot-rolledsteel sheet can be suppressed.

[Cold-Rolling Process]

In the cold-rolling process, the hot-rolled steel sheet is cold-rolledsuch that a cumulative rolling reduction R_(CR) is 70% to 90% to obtaina cold-rolled steel sheet. By cold-rolling the hot-rolled steel sheet towhich the predetermined residual stress is applied at theabove-described cumulative rolling reduction, ferrite having thepredetermined texture can be obtained after annealing and cooling.

When the cumulative rolling reduction is less than 70%, the texture ofthe cold-rolled sheet (cold-rolled steel sheet) is not sufficientlydeveloped. Therefore, the predetermined texture cannot be obtained afterannealing. In addition, when the cumulative rolling reduction is morethan 90%, the texture of the cold-rolled sheet is excessively developed.Therefore, the predetermined texture cannot be obtained after annealing.In addition, the rolling force increases, and the uniformity of thematerial in the sheet width direction deteriorates. Further, theproduction stability also deteriorates. Therefore, the cumulativerolling reduction R_(CR) during cold-rolling is set to be 70% to 90%.

[Annealing Process]

In the annealing process, the cold-rolled steel sheet is heated to thesoaking temperature at the average heating rate corresponding to theresidual stress applied in the stress application process and thecumulative rolling reduction in the cold-rolling process, and is held atthe soaking temperature corresponding to the residual stress applied inthe stress application process and the cumulative rolling reduction inthe cold-rolling process.

Specifically, in the annealing process, the cold-rolled steel sheet isheated such that an average heating rate in a range from 300° C. to asoaking temperature T1° C. that satisfies the following Expression (1)is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at thesoaking temperature T1° C. for 30 seconds to 150 seconds for annealing.

1275−27×1n(σs)−4.5×R _(CR) ≤T1≤1275−27×1n(σs)−4×R _(CR)   (1)

When the average heating rate is slower than 1.5° C./sec, a long periodof time is required for heating, and the productivity deteriorates,which is not preferable. In addition, when the average heating rate isfaster than 10.0° C./sec, the uniformity of the temperature in the sheetwidth direction deteriorates, which is not preferable.

In addition, when the soaking temperature T1 is lower than1275−27×1n(σs)−4.5×R_(CR), although recrystallization of ferrite andreversible transformation from ferrite to austenite do not sufficientlyprogress, and the predetermined texture cannot be obtained. In addition,inhomogeneous deformation during forming is promoted due to a differencein strength between non-recrystallized crystal grains and recrystallizedcrystal grains, which is not preferable. In addition, when the soakingtemperature is higher than 1275−27×1n(σs)−4×R_(CR), althoughrecrystallization of ferrite and reversible transformation from ferriteto austenite sufficiently progresses, crystal grains are coarsened, andthe predetermined texture cannot be obtained, which is not preferable.

The average heating rate can be obtained from (Heating EndTemperature−Heating Start Temperature)/(Heating Time).

[Cooling Process]

In the cooling process, the cold-rolled steel sheet after soaking in theannealing process is cooled. During cooling, the cold-rolled steel sheetis cooled to 550° C. to 650° C. such that an average cooling rate in arange from T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec and iscooled to 200° C. to 490° C. such that the average cooling rate is 5.0°C./sec to 500.0° C./sec.

When the average cooling rate in a range from T1° C. to 650° C. isslower than 1.0° C./sec, ferritic transformation is excessivelypromoted, and the predetermined volume fraction of the secondary phasecannot be obtained. On the other hand, when the average cooling rate ina range from T1° C. to 650° C. is faster than 10.0° C./sec, ferritictransformation do not sufficiently progress, and concentration of carbonon austenite does not sufficiently progress. Therefore, thepredetermined volume fraction of the secondary phase cannot be obtained.

In addition, when the average cooling rate from this temperature rangeto a temperature range of 200° C. to 490° C. after cooling is performedin a temperature range of 550° C. to 650° C. is slower than 5.0° C./sec,ferritic transformation is excessively promoted. Therefore, thepredetermined volume fraction of the secondary phase cannot be obtained.On the other hand, it is difficult to set the average cooling rate to befaster than 500.0° C./sec due to the facility restriction. Therefore,the upper limit may be 500.0° C./sec.

The average cooling rate can be obtained from (Cooling StartTemperature−Cooling End Temperature)/(Cooling Time).

[Holding Process]

The cold-rolled steel sheet that is cooled to 200° C. to 490° C. may beheld in the temperature range of 200° C. to 490° C. for 30 to 600seconds.

By holding the cold-rolled steel sheet in the temperature range for thepredetermined time, the effect of improving ductility through temperingmartensite can be obtained, which is preferable.

The cold-rolled steel sheet that is cooled to 200° C. to 490° C. or thecold-rolled steel sheet after the holding process may be cooled to roomtemperature at 10° C./sec or faster.

A plating process of forming a plating layer on the surface may befurther performed on the cold-rolled steel sheet obtained using theabove-described method. Examples of the plating process include thefollowing process.

[Electroplating Process]

[Galvannealing Process]

The cold-rolled steel sheet after the cooling process or the holdingprocess may be electroplated to form an electroplating layer on thesurface. An electroplating method is not particularly limited. Theelectroplating method may be determined depending on requiredcharacteristics (for example, corrosion resistance or adhesion).

In addition, after electroplating, the cold-rolled steel sheet may beheated to alloy plating metal.

[Hot-Dip Galvanizing Process]

[Galvannealing Process]

The cold-rolled steel sheet after the cooling process or the holdingprocess may be hot-dip galvanized to form a hot-dip galvanized layer onthe surface. A hot-dip galvanizing method is not particularly limited.The hot-dip galvanizing method may be determined depending on requiredcharacteristics (for example, corrosion resistance or adhesion).

In addition, the cold-rolled steel sheet after hot-dip galvanizing maybe heat-treated to alloy a plating layer. In a case where alloying isperformed, it is preferable that the cold-rolled steel sheet isheat-treated in a temperature range of 400° C. to 550° C. for 3 to 60seconds.

With the above-described manufacturing method, the steel sheet accordingto the embodiment can be obtained.

EXAMPLES

Next, examples of the present invention will be described. However,conditions of the examples are merely exemplary to confirm theoperability and the effects of the present invention, and the presentinvention is not limited to these condition examples. The presentinvention can adopt various conditions within a range not departing fromthe scope of the present invention as long as the object of the presentinvention can be achieved under the conditions.

Steels having chemical compositions shown in “Slab No. A to Z” of Table1 were melted, and slabs having a thickness of 240 to 300 mm weremanufactured by continuous casting. The obtained slabs were heated at atemperature shown in Tables 2A to 2C. The heated slabs were hot-rolledunder conditions shown in Tables 2A to 2C and were coiled.

Next, the coils were uncoiled and a stress was applied to the hot-rolledsteel sheets. At this time, while measuring the surface layer residualstress on-line using a portable X-ray residual stress analyzer at aworking temperature (steel sheet temperature) shown in Tables 2A to 2C,a contact pressure of a grinding brush on the steel sheet surface waschanged such that the residual stress was as shown in Tables 2A to 2C.

Next, by performing cold rolling at a rolling reduction (cumulativerolling reduction) shown in Tables 2A to 2C, steel sheets Al to Z1 wereobtained.

Next, the steel sheets were annealed under conditions shown in Tables 3Ato 3F, were cooled to 500° C. to 650° C. at cooling rates shown in thetables, and were further cooled to temperatures shown in the tables. Aholding process of holding the steel sheet at 200° C. to 490° C. for 30to 600 seconds was further performed on some steel sheets. After coolingor holding, the steel sheets were air-cooled to room temperature. Next,some steel sheets were plated in various ways to form a plating layer onthe surface. In the tables, CR represents that no plating was performed,GI represents that hot-dip galvanizing was performed, GA represents thathot-dip galvannealing was performed, EG represents that electroplatingwas performed, EGA represents that electrogalvannealing was performed,and Zn-Al-Mg or the like represents that plating including theseelements was performed.

Regarding each of the product sheets No. A1a to Z1a, the observation ofthe metallographic structures in the surface layer region and theinternal region and the measurement of X_(ODF{001}/{111}) wereperformed.

In addition, in a range of 0 to 50 μm from the surface in the sheetthickness direction, the average number density and the minimum numberdensity of the secondary phase per observed viewing field having alength of 100 μm in the sheet width direction and a length of 50 μm inthe sheet thickness direction were obtained.

The volume fraction of the secondary phase in the surface layer regionwas obtained using the following method.

A sample (20 mm in the rolling direction×20 mm in the widthdirection×the thickness of the steel sheet) for metallographic structure(microstructure) observation was collected from a W/4 position of asheet width W of the obtained steel sheet, and a metallographicstructure in a range from the surface layer to the ¼ thickness positionwas observed using an optical microscope to calculate the area fractionof the secondary phase in a range from the surface of the steel sheet(in a case where a plating layer was present, the surface excluding theplating layer) to 20 μm. In order to prepare the sample, a sheetthickness cross section in an orthogonal-to-rolling direction waspolished as an observation section and was etched with the LePerareagent.

“Microstructures” were classified based on an optical microscope imageat a magnification by 500-times obtained after etching with the LePerareagent. A region ranging from the surface layer to a ¼ thicknessposition in the steel sheet etched with the LePera reagent was observedin 10 viewing fields at a magnification by 500-times, a region from thesurface layer to a position of 20 μm of the steel sheet in the structureimage is designated, and the image was processed using image analysissoftware “Photoshop CS5” (manufactured by Adobe Inc.) to obtain the areafraction of the secondary phase. By performing the image analysis asdescribed above in 10 viewing fields in total, the area fraction of thesecondary phase was measured. Further, the average of the area fractionvalues was calculated as the volume fraction of the secondary phase inthe surface layer region.

In addition, the average grain size of the secondary phase in thesurface layer region was obtained using the following method.

Using the same method as the method for obtaining the volume fraction ofthe secondary phase, a region ranging from the surface layer to a ¼thickness position in the steel sheet etched with the LePera reagent wasobserved in 10 viewing fields at a magnification by 500-times, a regionfrom the surface layer to a position of 20 μm×200 μm of the steel sheetin the structure image was selected, and the image was processed usingimage analysis software “Photoshop CS5” (manufactured by Adobe Inc.) tocalculate the area of the secondary phase and the number of grains ofthe secondary phase. By adding up the values and dividing the area ofthe secondary phase by the number grains of the secondary phase, theaverage area per grain of the secondary phase was calculated. The circleequivalent diameter was calculated based on the area and the number ofgrains to obtain the average grain size of the secondary phase.

The volume fraction and the average grain size of the secondary phase inthe internal region was obtained by using a steel sheet etched with theLePera reagent, selecting a range from a position of more than 20 μmfrom the surface of the sample in the sheet thickness direction to a ¼thickness position, and analyzing the range with the same method as thatof the surface layer region.

X_(ODF{001}/{111}) in ferrite of the surface layer region was obtainedin the following manner using EBSD (Electron Backscattering Diffraction)method.

Regarding a sample provided for EBSD method, the steel sheet waspolished by mechanical grinding, strain was removed by chemicalpolishing or electrolytic polishing, the sample was prepared such thatthe cross section in the sheet thickness direction including the rangefrom the surface to the ¼ thickness position was a measurement surface,and the texture was measured. Regarding a sample collection position inthe sheet width direction, the sample was collected at a sheet widthposition of W/4 (position at a distance of ¼ from an end surface of thesteel sheet in the sheet width direction).

In the region of the sample ranging from the surface of the steel sheetto 20 μm in the sheet thickness direction, a crystal orientationdistribution was measured by EBSD method at a pitch of 0.5 μm or less.Ferrite was extracted using an IQ (Image Quality) map that is analyzableby EBSP-OIM (registered trade name, Electron Backscatter DiffractionPattern-Orientation Image Microscopy). The threshold of the IQ value wasset such that the area fraction of ferrite that was calculated by theobservation of the microstructure obtained by the LePera corrosionmatched the area fraction of ferrite calculated based on the IQ value.In a cross section of Φ2=45° in a three-dimensional texture (ODF:Orientation Distribution Functions) calculated using crystalorientations of the extracted ferrite, the ratio of a maximum value ofX-ray random intensity ratios of a {001} orientation group to a maximumvalue of X-ray random intensity ratios of a {111} orientation group(y-fiber) was obtained as X_(ODF{001}/{111}).

In addition, a texture of ferrite in the internal region was obtained bydesignating a range from a position of more than 20 μm from the surfaceof the sample in the sheet thickness direction to a ¼ thickness positionby the above-described EBSD method and analyzing the range with the samemethod as that of the surface layer region.

The number density of the grains of the secondary phase and theunevenness in the number density in the range of 0 to 50 μm from thesurface in the sheet thickness direction were obtained by using a steelsheet etched with the LePera reagent, designating 10 or more observedviewing fields of 100 μm (sheet width direction)×50 (sheet thicknessdirection) in a range from the surface of the sample to a position of 50μm from the surface in the sheet thickness direction, and counting thenumber of grains of the secondary phase in the observed viewing fields.The results are shown in Tables 4A, 4B and 4C.

The tensile strength was obtained in a tensile test that was performedaccording to JIS Z 2241 using a JIS No. 5 test piece cut from thedirection orthogonal to the rolling direction. The results are shown inTables 3A to 3F.

[Evaluation of Surface Appearance Quality of Steel Sheet]

In addition, regarding each of the manufactured product sheets, thesurface appearance quality of the steel sheet was evaluated.

Specifically, the surface of the manufactured steel sheet was observedby viewing inspection to evaluate the surface state. The evaluationcriteria of the surface appearance quality of the steel sheet were asfollows.

A: no pattern was formed (more desirably, can be used as an exteriormaterial)

B: an acceptable small pattern was formed (can be used as an exteriormaterial)

C: an unacceptable pattern was formed (cannot be used as an exteriormaterial)

D: a significant pattern defect was formed (cannot be used as acomponent)

The results are shown in Tables 3A to 3F.

[Forming Test of Steel Sheet]

A forming test was not performed on the material for which the surfaceappearance quality of the steel sheet was evaluated as C or D, and theforming test was performed only on the material for which the surfaceappearance quality of the steel sheet was evaluated as A or B.

Regarding forming, plastic strain of 10% in the rolling width directionwas applied to the steel sheet of which the surface properties wasmeasured in a cylinder drawing forming test with the Marciniak methodusing a deep drawing tester, a cylindrical punch of ϕ50 mm, and acylindrical die of ϕ54 mm.

A test piece of 100 mm in the rolling width direction×50 mm in therolling direction was prepared from a portion deformed by forming, andan arithmetic mean height Pa of a profile curve defined by JIS B0601(2001) was measured in the direction orthogonal to the rolling directionaccording to JIS B0633 (2001). The evaluation was performed in theportion deformed by forming, and the evaluation length was 30 mm.

A test piece of 100 mm in the width direction×50 mm in the rollingdirection was prepared from a flat portion of the formed article, and anarithmetic mean height Pa of a profile curve defined by JIS B0601 (2001)was measured in the direction orthogonal to the rolling directionaccording to JIS B0633 (2001). The evaluation length was 30 mm.

The amount ΔPa of increase in roughness was calculated using Pa of theformed article and Pa of the steel sheet obtained in the above-describedmeasurement test.

Amount ΔPa of Increase in Roughness=Pa of Formed Article−Pa of SteelSheet

The surface properties of the steel sheet after forming were evaluatedbased on the ΔPa. The evaluation criteria were as follows.

A: ΔPa≤0.25 μm (more desirably, can be used as an exterior material)

B: 0.25 μm<ΔPa≤0.35 μm (can be used as an exterior material)

C: 0.35 μm <ΔPa≤0.55 μm (can be used as a component but cannot be usedas an exterior material)

D: 0.55 μm<ΔPa (cannot be used as a component)

[Comprehensive Evaluation]

Regarding evaluation criteria of the surface properties, among theabove-described evaluation results (the evaluation of the steel sheetand the evaluation after forming), an evaluation result having a lowerscore was obtained as the comprehensive evaluation. The results areshown in Tables 4A to 4C.

A: more desirably, the material can be used as an exterior material

B: the material can be used as an exterior material

C: the material cannot be used as an exterior material

D: the material cannot be used as a component

Based on Tables 1 to 4C, in the examples (examples according to thepresent invention) where the chemical composition, the metallographicstructure in the surface layer region, the metallographic structure inthe internal region, and X_(ODF{001}/{111}) were in the preferableranges, the comprehensive evaluation was A or B, and the formation ofsurface unevenness after working was suppressed.

On the other hand, in the examples (comparative examples) where one ormore of the chemical composition, the metallographic structure in thesurface layer region, the metallographic structure in the internalregion, and X_(ODF{001}/{111}) were outside of the ranges according tothe present invention, a pattern was formed or unevenness occurred inthe stage of the steel sheet or after forming such that the material wasnot able to be used as an exterior material.

TABLE 1 Slab Chemical Composition mass % (Remainder: Fe + Impurities)No. C Si Mn P S Al N Ti Nb B Mo Cr Ni A 0.051 0.050 0.45 0.020 0.0050.20 0.0030 0.000 0.000 0.0025 0.39 0.52 0.00 B 0.040 0.050 1.05 0.0050.004 0.11 0.0022 0.000 0.000 0.0000 0.29 0.31 0.00 C 0.053 0.030 1.740.020 0.006 0.14 0.0025 0.000 0.000 0.0000 0.00 0.40 0.00 D 0.042 0.0071.33 0.007 0.004 0.02 0.0030 0.002 0.000 0.0000 0.00 0.53 0.00 E 0.0900.200 0.85 0.015 0.006 0.01 0.0100 0.000 0.000 0.0000 0.00 0.00 0.00 F0.070 0.040 1.15 0.015 0.006 0.45 0.0030 0.000 0.000 0.0015 0.00 0.000.00 G 0.055 0.100 1.68 0.030 0.008 0.06 0.0026 0.000 0.000 0.0018 0.010.14 0.00 H 0.020 0.010 2.10 0.016 0.004 0.03 0.0035 0.000 0.000 0.00000.00 0.06 0.00 I 0.030 0.005 1.83 0.025 0.006 0.08 0.0020 0.000 0.0000.0025 0.01 0.20 0.01 J 0.035 0.008 1.77 0.020 0.006 0.08 0.0020 0.0000.000 0.0004 0.00 0.20 0.25 K 0.060 0.040 0.95 0.015 0.006 0.45 0.00300.000 0.000 0.0015 0.15 0.10 0.01 L 0.030 0.006 1.88 0.015 0.007 0.050.0020 0.000 0.000 0.0015 0.00 0.20 0.00 M 0.053 0.030 1.64 0.020 0.0060.14 0.0025 0.000 0.000 0.0006 0.00 0.40 0.00 N 0.007 0.010 0.10 0.0150.005 0.05 0.0035 0.050 0.001 0.0008 0.00 0.00 0.00 O 0.016 0.020 1.800.030 0.005 0.03 0.0015 0.000 0.000 0.0027 0.01 0.05 0.00 P 0.100 0.1001.64 0.028 0.006 0.05 0.0016 0.000 0.000 0.0018 0.01 0.14 0.00 Q 0.0950.350 1.60 0.020 0.005 0.05 0.0010 0.000 0.000 0.0022 0.01 0.15 0.00 R0.037 0.010 0.19 0.010 0.005 0.03 0.0028 0.001 0.000 0.0001 0.00 0.000.00 S 0.020 0.030 2.20 0.020 0.006 0.08 0.0025 0.000 0.000 0.0019 0.000.10 0.00 T 0.037 0.050 2.00 0.008 0.005 0.60 0.0030 0.010 0.000 0.00000.00 0.50 0.00 U 0.055 0.100 1.68 0.030 0.008 0.06 0.0026 0.000 0.0000.0051 0.01 0.10 0.00 V 0.020 0.020 1.40 0.050 0.004 0.04 0.0030 0.0500.020 0.0007 0.00 0.10 0.00 W 0.060 0.017 1.21 0.015 0.003 0.02 0.00310.000 0.000 0.0000 0.42 0.00 0.00 X 0.085 0.011 1.33 0.004 0.002 0.120.0045 0.000 0.000 0.0000 0.35 0.80 0.00 Z 0.070 0.250 1.70 0.020 0.0050.40 0.0010 0.000 0.000 0.0022 0.01 0.00 0.00

TABLE 2A Heating Hot-Rolling Process Coiling Cold-Rolling Process FinishRolling Finish Rolling Temperature Process Stress Application ProcessProcess Heating Start Finishing Change in Hot- Coiling Residual WorkingRolling Steel Temperature Temperature Temperature Rolling ProcessTemperature Stress Temperature T Reduction Slab Sheet ° C. ° C. ° C. °C. ° C. MPa ° C. % A A1 1200 950 890 −60 700 248 112  78 A A2 1200 950890 −60 750 325 40 85 A A3 1200 950 890 −60 750 248 112  92 A A4 1200940 880 −60 660 26 165  80 A A5 1220 1010 930 −60 620 12 140  80 B B11200 930 880 −50 600 104 30 77 B B2 1100 850 880 30 550 177 242  85 B B31200 810 850 40 680 265 43 85 B B4 1200 930 880 −50 600 180 30 80 C C11200 910 890 −20 600 205 30 85 C C2 1200 845 870 35 580 337 103  80 C C31050 800 850 50 600 424 40 90 C C4 1050 800 850 50 600 223 30 72 C C51050 800 850 50 600 10 *2 72 C C6 1230 1020 920 −100 640 32 120  85 D D11100 850 885 35 530 153 30 90 D D2 1100 850 885 35 480 349 30 87 D D31100 850 885 35 480 349 30 92 E E1 1300 1080 950 −130 600 238 120  85 EE2 1280 1050 930 −120 550 181 50 85 *2 represents that stressapplication was not performed.

TABLE 2B Heating Hot-Rolling Process Coiling Cold-Rolling Process FinishRolling Finish Rolling Temperature Process Stress Application ProcessProcess Heating Start Finishing Change in Hot- Coiling Residual WorkingRolling Steel Temperature Temperature Temperature Rolling ProcessTemperature Stress Temperature T Reduction Slab Sheet ° C. ° C. ° C. °C. ° C. MPa ° C. % E E3 1100 860 887 27 640 210 40 87 E E4 1100 860 88727 640 151 40 90 F F1 1200 950 900 −50 680 204 27 80 F F2 1200 950 900−50 680 426 25 73 G G1 1100 850 885 35 700 231 30 85 G G2 1100 850 88535 700 80 510  87 G G3 1100 850 885 35 700 87 510  68 H H1 1200 930 890−40 560 161 45 85 H H2 1200 930 890 −40 560 164 45 65 H H3 1300 1090 960−130 700 161 45 85 I I1 1200 850 890 40 680 197 30 82 I I2 1200 850 89040 680 10 *2 82 J J1 1200 910 890 −20 700 302 35 83 J J2 1100 850 870 20650 250 30 72 J J3 1010 790 860 70 550 287 27 83 J J4 1200 910 890 −20760 260 50 83 K K1 1200 920 890 −30 600 301 20 88 K K2 1200 930 870 −60650 275 35 87 K K3 1200 820 845 25 700 375 20 85 K K4 1200 820 845 25700 245 25 82 *2 represents that stress application was not performed.

TABLE 2C Heating Hot-Rolling Process Coiling Cold-Rolling Process FinishRolling Finish Rolling Temperature Process Stress Application ProcessProcess Heating Start Finishing Change in Hot- Coiling Residual WorkingRolling Steel Temperature Temperature Temperature Rolling ProcessTemperature Stress Temperature T Reduction Slab Sheet ° C. ° C. ° C. °C. ° C. MPa ° C. % L L1 1250 850 880 30 580 355 50 85 L L2 1250 850 88030 580 313 25 82 M M1 1200 925 895 −30 520 250 110  87 M M2 1200 925 895−30 520 200 110  92 N N1 1250 960 910 −50 760 466 30 87 O O1 1200 925870 −55 480 329 45 72 P P1 1100 860 865 5 670 10 *2 80 P P2 1200 950 890−60 700 335 40 80 Q Q1 1200 950 905 −45 650 198 40 72 R R1 1200 920 890−30 550 10 *2 80 S S1 1200 930 880 −50 500 215 55 80 S S2 1200 930 880−50 500 185 300  92 T T1 1100 835 870 35 600 211 30 77 U U1 1050 800 85050 650 220 208  77 U U2 1050 800 850 50 650 220 208  68 V V1 1200 910880 −30 700 221 50 70 W W1 1250 950 905 −45 550 311 30 65 W W2 1250 950905 −45 550 161 45 85 X X1 1200 920 880 −40 650 150 40 85 Z Z1 1100 860887 27 640 210 40 87 *2 represents that stress application was notperformed.

TABLE 3A Annealing Process Cooling Process Lower Upper An- CoolingCooling Cooling Cooling Prod- Average Limit of Limit of nealing An- RateStop Rate in Stop Surface Sheet uct Heating Expres- Expres- Temper-nealing in T1 to Temper- 200° C. Temper- Treatment Thick- Steel SheetRate sion sion ature Time 650° C. ature to 490° C. ature Holding Type ofness Sheet No. ° C./s (1) (1) ° C. sec ° C./s ° C. ° C./s ° C. StepPlating mm A1 A1a 3.7 775 814 790 90.0 4.3 560 10 450 Not Provided GA0.50 A1 A1b 3.7 775 814 790 90.0 4.3 570 10 450 Not Provided Lubricant0.50 GA A2 A2a 2.0 736 779 770 140.0 2.9 560 7 420 Not Provided GI 0.35A2 A2b 2.0 736 779 770 140.0 2.9 560 7 450 Not Provided Al—Si 0.35 A2A2c 2.0 736 779 770 140.0 2.9 580 7 450 Not Provided Zn—Al 0.35 A3 A3a4.2 712 758 770 80.0 4.8 560 14 400 Not Provided GA 0.25 A4 A4a 3.6 827867 850 66.0 2.6 570 6 460 Not Provided GA 0.40 A5 A5a 3.9 848 888 860130.0 3.4 590 8 470 Not Provided GA 0.40 B1 B1a 3.2 803 842 840 110.03.6 560 9 400 Not Provided GA 0.75 B2 B2a 2.5 753 795 760 130.0 2.9 5807 490 Not Provided GA 0.40 B3 B3a 5.8 742 784 750 60.0 6.2 590 16 470Not Provided GA 0.45 B4 B4a 2.7 775 815 800 120.0 3.1 620 8 460 NotProvided GA 0.45 B4 B4b 2.7 775 815 800 120.0 3.1 560 8 450 Not ProvidedZn—Al—Mg 0.45 C1 C1a 5.8 749 791 780 60.0 6.2 600 16 400 Not Provided GA0.35 C2 C2a 3.7 758 798 780 90.0 4.3 580 10 400 Provided GA 0.65 C3 C3a2.7 707 752 750 120.0 3.1 560 8 350 Not Provided GI 0.22 C4 C4a 5.8 805841 820 60.0 6.2 550 16 350 Provided GI 0.55 C5 C5a 9.5 889 925 860 30.09.9 570 80 400 Not Provided GA 0.55 C6 C6a 3.2 799 841 820 70.0 2.6 5806 450 Not Provided GA 0.40 D1 D1a 5.8 734 779 770 60.0 6.2 560 40 400Not Provided GA 0.30 D2 D2a 3.3 725 769 765 100.0 3.8 590 15 400Provided GA 0.40 D3 D3a 2.7 703 749 745 60.0 3.1 600 8 350 Not ProvidedGI 0.25 E1 E1a 3.7 745 787 780 90.0 4.3 550 10 350 Provided GI 0.45

TABLE 3B Annealing Process Cooling Process Lower Upper An- CoolingCooling Cooling Cooling Prod- Average Limit of Limit of nealing An- RateStop Rate in Stop Surface Sheet uct Heating Expres- Expres- Temper-nealing in T1 to Temper- 200° C. Temper- Treatment Thick- Steel SheetRate sion sion ature Time 650° C. ature to 490° C. ature Holding Type ofness Sheet No. ° C./s (1) (1) ° C. sec ° C./s ° C. ° C./s ° C. StepPlating mm E2 E2a 2.7 752 795 780 120.0 3.1 610 8 450 Not GA 0.45Provided E3 E3a 2.7 739 783 780 120.0 3.1 590 8 450 Not GA 0.40 ProvidedE3 E3b 2.7 739 783 780 120.0 3.1 560 8 400 Provided Zn—Al—Mg—Si 0.40 E4E4a 5.4 739 783 780 60.0 6.2 580 40 200 Provided Sn 0.15 F1 F1a 6.6 771811 800 50.0 7.6 560 100 200 Provided CR 0.60 F1 F1b 6.6 771 811 80050.0 7.6 570 100 250 Provided Phosphate 0.60 Coating EG F2 F2a 8.3 783820 820 40.0 9.2 580 150 250 Provided CR 0.80 G1 G1a 3.8 746 788 78070.0 6.7 560 80 250 Provided EG 0.50 G2 G2a 5.4 765 809 770 50.0 9.5 57070 250 Provided EG 0.40 G3 G3a 2.5 848 882 850 110.0 4.3 580 20 250Provided CR 0.95 H1 H1a 1.7 755 798 790 150.0 3.0 580 10 300 Provided CR0.50 H1 H1b 1.7 755 798 750 150.0 3.0 560 10 300 Provided CR 0.50 H2 H2a3.4 845 877 820 80.0 6.0 570 20 300 Provided CR 1.00 H3 H3a 1.7 755 798790 150.0 3.0 580 10 300 Provided CR 1.00 I1 I1a 6.6 763 804 780 50.07.6 590 100 300 Provided CR 0.40 I2 I2a 2.2 844 885 820 120.0 3.9 600 50300 Provided CR 0.40 J1 Ha 2.9 747 789 780 90.0 5.2 560 24 400 ProvidedCR 0.55 J2 J2a 2.5 802 838 835 110.0 4.3 600 20 200 Not CR 0.85 ProvidedJ3 J3a 3.4 749 790 780 80.0 6.0 580 30 400 Provided EG 0.55 J4 J4a 2.2751 793 780 120.0 3.9 570 18 200 Not EG 0.55 Provided K1 K1a 2.2 725 769760 120.0 3.9 570 18 200 Not EG 0.35 Provided K2 K2a 2.2 732 775 760120.0 3.9 580 18 350 Provided CR 0.40 K3 K3a 4.4 732 775 800 60.0 7.7560 35 350 Provided CR 0.35

TABLE 3C Annealing Process Cooling Process Lower Upper An- CoolingCooling Cooling Cooling Prod- Average Limit of Limit of nealing An- RateStop Rate in Stop Surface Sheet uct Heating Expres- Expres- Temper-nealing in T1 to Temper- 200° C. Temper- Treatment Thick- Steel SheetRate sion sion ature Time 650° C. ature to 490° C. ature Holding Type ofness Sheet No. ° C./s (1) (1) ° C. sec ° C./s ° C. ° C./s ° C. StepPlating mm K4 K4a 2.9 757 798 780 90.0 5.2 570 25 350 Provided EG 0.40L1 L1a 3.7 734 776 780 90.0 4.3 560 20 250 Not Provided GA 0.35 L2 L2a9.3 751 792 780 150.0 9.9 580 500 200 Not Provided EGA 0.55 L2 L2b 2.0751 792 780 150.0 1.1 600 50 460 Not Provided GA 0.55 L2 L2c 11.5 751792 780 30.0 9.9 560 500 460 Not Provided GA 0.55 M1 M1a 3.7 734 778 77090.0 4.3 550 20 460 Not Provided GA 0.40 M2 M2a 3.7 718 764 770 90.0 4.3580 20 450 Not Provided GA 0.20 N1 N1a 4.4 718 761 750 60.0 7.7 590 35400 Provided CR 0.30 O1 O1a 4.2 795 831 810 80.0 4.8 560 20 430 NotProvided GA 0.65 P1 P1a 5.4 853 893 800 60.0 6.2 600 32 430 Not ProvidedGA 0.60 P2 P2a 3.8 758 798 790 100.0 5.4 570 25 450 Not Provided GA 0.60Q1 Q1a 2.7 808 844 840 120.0 3.1 560 16 450 Not Provided GA 0.85 R1 R1a2.7 853 893 860 120.0 3.1 550 16 400 Not Provided GI 0.55 S1 S1a 3.7 770810 800 90.0 4.4 560 19 350 Provided CR 0.60 S2 S2a 3.7 720 766 780 90.04.3 580 18 445 Not Provided GA 0.20 T1 T1a 3.8 784 822 790 100.0 5.4 59025 445 Not Provided GA 0.75 U1 U1a 5.4 783 821 800 60.0 6.2 600 30 420Not Provided GI 0.60 U2 U2a 4.7 823 857 800 60.0 7.7 610 35 300 ProvidedCR 0.70 V1 V1a 2.9 814 849 810 90.0 5.2 560 25 450 Not Provided GA 0.95W1 W1a 2.9 828 860 850 90.0 5.2 570 25 300 Provided CR 1.10 W2 W2a 2.9755 798 790 120.0 3.1 590 15 300 Provided CR 0.45 X1 X1a 2.7 757 800 790150.0 3.1 600 8 450 Not Provided GA 0.35 Z1 Z1a 2.2 739 783 760 120.03.9 610 15 450 Not Provided GA 0.40

TABLE 3D Surface Layer Region Internal Region ΔXODF Average GrainAverage Grain {001}/{111} Fraction of Size of XODF Fraction of Size ofXODF Surface Layer Surface Secondary Secondary {001}/ SecondarySecondary {001}/ Region- Evaluation Tensile Product Phase Phase {111} ofPhase Phase {111} of Internal (Steel Strength Sheet No. % μm Ferrite %μm Ferrite Region Sheet) MPa Note A1a 1.5 1.1 1.90 4.9 1.5 1.50 0.40 A431 Example A1b 1.5 1.1 1.90 4.9 1.5 1.50 0.40 A 461 Example A2a 1.7 3.21.70 4.4 3.5 1.40 0.30 A 455 Example A2b 1.3 2.9 1.70 4.0 3.3 1.40 0.30B 450 Example A2c 1.4 3.0 1.70 4.1 3.5 1.40 0.30 B 451 Example A3a 1.92.1 2.50 4.1 3.5 0.30 2.20 B 452 Comparative Example A4a 6.6 2.6 1.701.8 2.3 1.60 0.10 B 426 Comparative Example A5a 1.6 2.9 1.82 5.1 4.31.78 0.04 C 457 Comparative Example B1a 0.9 3.5 0.51 4.8 2.8 0.48 0.03 B459 Comparative Example B2a 2.8 1.5 1.40 5.8 1.8 1.10 0.30 A 473 ExampleB3a 2.1 1.1 0.80 6.4 1.9 0.66 0.14 A 479 Example B4a 1.3 2.1 0.65 3.82.4 0.45 0.20 A 447 Example B4b 1.1 2.0 0.65 3.4 2.5 0.45 0.20 A 442Example C1a 4.8 1.1 0.68 7.1 1.8 0.54 0.14 A 491 Example C2a 4.5 0.80.73 8.0 1.4 0.51 0.22 A 502 Example C3a 4.4 2.2 3.20 8.5 1.3 0.50 2.70A 508 Comparative Example C4a 4.9 2.6 0.85 7.4 2.8 0.71 0.14 A 495Example C5a 5.1 0.4 0.33 9.4 3.9 0.25 0.08 A 519 Comparative Example C6a1.9 2.2 1.50 1.9 2.4 1.50 0.00 A 412 Comparative Example D1a 4.3 3.31.80 9.7 3.5 1.50 0.30 A 522 Example D2a 4.7 2.5 1.60 8.8 3.0 1.38 0.22A 512 Example D3a 4.2 2.3 3.07 8.6 2.5 0.85 2.22 A 509 ComparativeExample E1a 0.3 0.08 0.83 3.4 1.1 0.75 0.08 A 441 Example

TABLE 3E Surface Layer Region Internal Region ΔXODF Average GrainAverage Grain {001}/{111} Fraction of Size of XODF Fraction of Size ofXODF Surface Layer Surface Secondary Secondary {001}/ SecondarySecondary {001}/ Region- Evaluation Tensile Product Phase Phase {111} ofPhase Phase {111} of Internal (Steel Strength Sheet No. % μm Ferrite %μm Ferrite Region Sheet) MPa Note E2a 0.2 0.05 0.81 3.0 1.2 0.68 0.13 A436 Example E3a 0.4 0.24 0.94 3.2 1.6 0.72 0.22 B 439 Example E3b 0.40.35 0.94 3.0 1.7 0.72 0.22 A 436 Example E4a 0.6 0.32 1.18 3.6 1.4 1.030.22 A 444 Example F1a 4.1 2.5 1.02 8.5 2.7 0.57 0.45 A 507 Example F1b4.1 2.5 1.02 8.5 2.7 0.57 0.45 A 507 Example F2a 4.8 4.5 0.55 10.6 4.50.32 0.23 A 533 Comparative Example G1a 3.9 1.5 1.40 6.9 2.4 1.00 0.40 B488 Example G2a 3.8 1.2 0.55 5.8 1.8 0.45 0.10 B 474 Comparative ExampleG3a 2.7 2.4 0.48 4.3 3.0 0.88 −0.40 B 455 Comparative Example H1a 1.51.6 0.65 4.1 2.0 0.59 0.06 A 451 Example H1b 1.1 1.2 0.49 3.6 1.5 0.67−0.18 A 445 Comparative Example H2a 0.5 1.9 0.20 2.5 2.3 0.30 −0.10 A431 Comparative Example H3a 1.9 3.1 0.41 3.8 3.4 0.37 0.04 A 448Comparative Example I1a 2.4 1.5 0.88 6.3 2.4 0.64 0.24 B 479 Example I2a2.6 1.8 0.24 5.9 2.9 0.22 0.02 B 474 Comparative Example J1a 0.5 1.41.00 4.9 1.1 0.77 0.23 B 459 Example J2a 0.3 0.7 1.10 4.8 1.1 0.73 0.37A 458 Example J3a 0.6 1.0 0.96 5.1 1.1 0.57 0.39 A 462 Example J4a 0.51.6 0.81 5.2 1.2 0.57 0.24 B 463 Example K1a 2.4 1.5 1.95 7.5 2.6 1.440.51 A 493 Example K2a 1.6 1.6 0.68 7.9 2.3 0.46 0.22 A 497 Example K3a1.9 2.0 4.60 7.6 2.2 3.10 1.50 A 493 Comparative Example

TABLE 3F Surface Layer Region Internal Region ΔXODF Average GrainAverage Grain {001}/{111} Fraction of Size of XODF Fraction of Size ofXODF Surface Laver Surface Secondary Secondary {001}/ SecondarySecondary {001}/ Region- Evaluation Pensile Product Phase Phase {111} ofPhase Phase {111} of Internal (Steel Strength Sheet No. % μm Ferrite %μm Ferrite Region Sheet) MPa Note K4a 1.7 1.1 1.13 7.1 1.6 1.28 −0.15 A487 Example L1a 4.5 2.1 2.50 6.8 2.2 2.54 −0.04 B 487 ComparativeExample L2a 3.7 1.9 1.70 6.9 2.1 1.48 0.22 B 488 Example L2b 1.4 2.21.65 5.7 1.7 1.64 0.01 A 470 Example L2c 5.1 1.1 0.55 5.2 1.4 0.58 −0.03A 469 Comparative Example M1a 2.8 0.99 0.62 4.0 1.5 0.41 0.21 A 452Example M2a 2.4 1.1 0.25 4.1 1.2 0.28 −0.03 A 452 Comparative ExampleN1a 0.005 0.1 0.30 0.2 0.2 0.07 0.23 A 352 Comparative Example O1a 2.61.6 0.55 3.7 2.1 0.24 0.31 A 448 Comparative Example P1a 5.5 0.7 0.3111.6 2.9 0.30 0.01 B 546 Comparative Example P2a 5.2 1.9 1.87 10.9 2.81.19 0.68 B 537 Comparative Example Q1a 12.1 3.1 0.48 9.7 2.1 0.56 −0.08C 532 Comparative Example R1a 0 — 0.45 0.3 0.1 0.18 0.27 A 314Comparative Example S1a 12.7 4.1 3.60 13.6 5.1 3.80 −0.20 B 580Comparative Example S2a 10.2 3.7 6.10 11.6 3.7 4.50 1.60 D 552Comparative Example T1a 11.9 2.0 1.50 18.9 4.6 1.80 −0.30 C 642Comparative Example U1a 5.1 2.2 2.35 5.3 1.8 0.84 1.51 C 470 ComparativeExample U2a 5.3 1.1 2.23 6.4 1.7 0.62 1.61 C 484 Comparative Example V1a1.2 0.7 0.11 1.1 0.9 0.17 −0.06 C 415 Comparative Example W1a 8.9 3.66.50 11.1 3.1 4.80 1.70 A 545 Comparative Example W2a 8.4 1.8 0.65 10.51.5 0.30 0.49 A 537 Comparative Example X1a 23.2 4.5 0.70 28.4 4.2 1.29−0.59 A 771 Comparative Example Z1a 16.5 4.2 0.62 21.0 5.1 0.19 0.43 B673 Comparative Example

TABLE 4A Surface to 50 μm Forming Test Difference from Amount ΔPaAverage Number Minimum Number of Increase Product Test Density ofDensity of Plastic in Roughness Surface Shape Comprehensive Sheet No.Piece No. Secondary Phase Secondary Phase Strain ε1 [μm] after FormingEvaluation Note A1a A1a-1 52 10 10% 0.27 B B Example A1b A1b-1 52 10 10%0.27 B B Example A2a A2a-1 30 9 10% 0.20 A A Example A2b A2b-1 41 9 10%0.16 A B Example A2c A2c-1 54 9 10% 0.17 A B Example A3a A3a-1 38 8 10%0.43 C C Comparative Example A4a A4a-1 46 10 10% 0.49 C C ComparativeExample A5a A5a-1 43 9 10% 0.41 C C Comparative Example B1a B1a-1 22 710% 0.43 C C Comparative Example B2a B2a-1 70 9 10% 0.16 A A Example B3aB3a-1 133 19 10% 0.27 B B Example B4a B4a-1 78 15 10% 0.28 B B ExampleB4b B4b-1 68 13 10% 0.25 A A Example C1a C1a-1 86 14 10% 0.27 B BExample C2a C2a-1 130 20 10% 0.26 B B Example C3a C3a-1 70 14 10% 0.48 CC Comparative Example C4a C4a-1 44 7 10% 0.28 B B Example C5a C5a-1 3510 10% 0.40 C C Comparative Example C6a C6a-1 48 9 10% 0.51 C CComparative Example D1a D1a-1 30 4 10% 0.32 B B Example D2a D2a-1 35 710% 0.25 A A Example D3a D3a-1 51 31 10% 0.46 C C Comparative Example

TABLE 4B Surface to 50 μm Forming Test Difference from Amount ΔPaAverage Number Minimum Number of Increase Product Test Density ofDensity of Plastic in Roughness Surface Shape Comprehensive Sheet No.Piece No. Secondary Phase Secondary Phase Strain ε1 [μm] after FormingEvaluation Note E1a E1a-1 55 10 10% 0.10 A A Example E2a E2a-1 85 20 10%0.10 A A Example E3a E3a-1 35 4 10% 0.09 A B Example E3b E3b-1 24 3 10%0.07 A A Example E4a E4a-1 28 3 10% 0.07 A A Example F1a F1a-1 50 12 10%0.20 A A Example F1b F1b-1 45 12 10% 0.18 A A Example F2a F2a-1 15 1010% 0.42 C C Comparative Example G1a G1a-1 70 11 10% 0.22 A B ExampleG2a G2a-1 73 16 10% 0.51 C C Comparative Example G3a G3a-1 28 8 10% 0.47C C Comparative Example H1a H1a-1 44 8 10% 0.27 B B Example H1b H1b-1 5621 10% 0.48 C C Comparative Example H2a H2a-1 85 30 10% 0.35 C CComparative Example H3a H3a-1 49 22 10% 0.46 C C Comparative Example I1aI1a-1 77 15 10% 0.23 A B Example I2a I2a-1 98 25 10% 0.37 C CComparative Example J1a J1a-1 35 10 10% 0.13 A B Example J2a J2a-1 12 310% 0.11 A A Example J3a J3a-1 15 5 10% 0.14 A A Example J4a J4a-1 20 610% 0.18 A B Example K1a K1a-1 51 7 10% 0.28 B B Example K2a K2a-1 66 2110% 0.29 B B Example

TABLE 4C Surface to 50 μm Forming Test Difference from Amount ΔPaAverage Number Minimum Number of Increase Product Test Density ofDensity of Plastic in Roughness Surface Shape Comprehensive Sheet No.Piece No. Secondary Phase Secondary Phase Strain ε1 [μm] after FormingEvaluation Note K3a K3a-1 47 16 10% 0.40 C C Comparative Example K4aK4a-1 39 8 10% 0.17 A A Example L1a L1a-1 51 11 10% 0.52 C C ComparativeExample L2a L2a-1 75 18 10% 0.25 B B Example L2b L2b-1 42 12 10% 0.16 AA Example L2c L2c-1 77 33 10% 0.40 C C Comparative Example M1a M1a-1 11813 10% 0.31 B B Example M2a M2a-1 94 6 10% 0.40 C C Comparative ExampleN1a N1a-1 2 2 10% 0.43 C C Comparative Example O1a O1a-1 77 18 10% 0.39C C Comparative Example P1a P1a-1 131 21 10% 0.48 C C ComparativeExample P2a P2a-1 101 29 10% 0.38 C C Comparative Example Q1a — 71 30 —— D D Comparative Example R1a R1a-1 — — 10% 0.37 C C Comparative ExampleS1a S1a-1 65 37 10% 0.64 D D Comparative Example S2a — 67 22 — — D DComparative Example T1a — 95 27 — — D D Comparative Example U1a — 60 24— — D D Comparative Example U2a — 57 26 — — D D Comparative Example V1a— 30 5 — — D D Comparative Example W1a W1a-1 51 15 10% 0.38 C CComparative Example W2a W2a-1 67 44 10% 0.36 C C Comparative Example X1aX1a-1 65 30 10% 0.40 C C Comparative Example Z1a Z1a-1 68 27 10% 0.43 CC Comparative Example

INDUSTRIAL APPLICABILITY

A high strength steel sheet in which the occurrence of surfaceunevenness is suppressed even after various deformation during pressforming can be manufactured. Therefore, the industrial applicability ishigh.

1. A steel sheet comprising, as a chemical composition, by mass %: C:0.020% to 0.090%; Si: 0.200% or less; Mn: 0.45% to 2.10%; P: 0.030% orless; S: 0.020% or less; sol. Al: 0.50% or less; N: 0.0100% or less; B:0% to 0.0050%; Mo: 0% to 0.40%; Ti: 0% to 0.10%; Nb: 0% to 0.10%; Cr: 0%to 0.55%; Ni: 0% to 0.25%; and a remainder of Fe and impurities, whereina metallographic structure in a surface layer region ranging from asurface to a position of 20 μm from the surface in a sheet thicknessdirection consists of ferrite and a secondary phase having a volumefraction of 0.01% to 5.0%, a metallographic structure in an internalregion ranging from a position of more than 20 μm from the surface inthe sheet thickness direction to a ¼ thickness position from the surfacein the sheet thickness direction consists of ferrite and a secondaryphase having a volume fraction of 2.0% to 10.0%, the volume fraction ofthe secondary phase in the surface layer region is less than the volumefraction of the secondary phase in the internal region, and in thesurface layer region, an average grain size of the secondary phase is0.01 μm to 4.0 μm, and a texture in which an X_(ODF{001}/{111}) as aratio of an intensity of {001} orientation to an intensity of {111}orientation in the ferrite is 0.60 or more and less than 2.00 isincluded.
 2. The steel sheet according to claim 1, wherein an averagegrain size of the secondary phase in the internal region is 1.0 μm to5.0 μm and is more than the average grain size of the secondary phase inthe surface layer region.
 3. The steel sheet according to claim 2,wherein in a range of 0 μm to 50 μm from the surface in the sheetthickness direction in a cross section of the steel sheet orthogonal toa rolling direction, an average number density of the secondary phaseper observed viewing field having a length of 100 μm in a sheet widthdirection and a length of 50 μm in the sheet thickness direction is 130or less, and a minimum number density of the secondary phase per theobserved viewing field is more than or equal to a value obtained bysubtracting 20 from the average number density of the secondary phase.4. The steel sheet according to claim 2, wherein the chemicalcomposition includes, by mass %, one or more selected from the groupconsisting of: B: 0.0001% to 0.0050%; Mo: 0.001% to 0.40%; Ti: 0.001% to0.10%; Nb: 0.001% to 0.10%; Cr: 0.001% to 0.55%; and Ni: 0.001% to0.25%.
 5. The steel sheet according to claim 2, wherein the secondaryphase in the surface layer region includes one or more selected from thegroup consisting of martensite, bainite, and tempered martensite.
 6. Thesteel sheet according to claim 2, wherein a plating layer is provided onthe surface.
 7. The steel sheet according to claim 2, wherein a tensilestrength is 400 MPa or higher.
 8. A method for manufacturing a steelsheet, the method comprising: a heating process of heating a slab havingthe chemical composition according to claim 1 at 1000° C. or higher; ahot-rolling process of hot-rolling the slab such that a rollingfinishing temperature is 950° C. or lower to obtain a hot-rolled steelsheet; a stress application process of applying a stress to thehot-rolled steel sheet after the hot-rolling process such that anabsolute value of a residual stress σs on a surface is 150 MPa to 350MPa; a cold-rolling process of cold-rolling the hot-rolled steel sheetafter the stress application process such that a cumulative rollingreduction R_(CR) is 70% to 90% to obtain a cold-rolled steel sheet; anannealing process of heating the cold-rolled steel sheet such that anaverage heating rate in a range from 300° C. to a soaking temperatureT1° C. that satisfies the following Expression (1) is 1.5° C./sec to10.0° C./sec and holding the heated steel sheet at the soakingtemperature T1° C. for 30 seconds to 150 seconds for annealing; and acooling process of cooling the cold-rolled steel sheet after theannealing process to 550° C. to 650° C. such that an average coolingrate in a range from T1° C. to 650° C. is 1.0° C./sec to 10.0° C./secand cooling the cooled steel sheet to 200° C. to 490° C. such that theaverage cooling rate is 5.0° C./sec to 500.0° C./sec,1275−27×1n(σs)−4.5×R _(CR) ≤T1≤1275−27×1n(σs)−4×R _(CR)   (1)
 9. Themethod for manufacturing a steel sheet according to claim 8, wherein thestress application process is performed at 40° C. to 500° C.
 10. Themethod for manufacturing a steel sheet according to claim 8, wherein inthe hot-rolling process, a finish rolling start temperature is 900° C.or lower.
 11. The method for manufacturing a steel sheet according toclaim 8, further comprising a holding process of holding the cold-rolledsteel sheet after the cooling process in a temperature range of 200° C.to 490° C. for 30 seconds to 600 seconds.
 12. A steel sheet comprising,as a chemical composition, by mass %: C: 0.020% to 0.090%; Si: 0.200% orless; Mn: 0.45% to 2.10%; P: 0.030% or less; S: 0.020% or less; sol. Al:0.50% or less; N: 0.0100% or less; B: 0% to 0.0050%; Mo: 0% to 0.40%;Ti: 0% to 0.10%; Nb: 0% to 0.10%; Cr: 0% to 0.55%; Ni: 0% to 0.25%; anda remainder of Fe and impurities, wherein a metallographic structure ina surface layer region ranging from a surface to a position of 20 μmfrom the surface in a sheet thickness direction comprises ferrite and asecondary phase having a volume fraction of 0.01% to 5.0%, ametallographic structure in an internal region ranging from a positionof more than 20 μm from the surface in the sheet thickness direction toa ¼ thickness position from the surface in the sheet thickness directioncomprises ferrite and a secondary phase having a volume fraction of 2.0%to 10.0%, the volume fraction of the secondary phase in the surfacelayer region is less than the volume fraction of the secondary phase inthe internal region, and in the surface layer region, an average grainsize of the secondary phase is 0.01 μm to 4.0 μm, and a texture in whichan X_(ODF{001}/{111}) as a ratio of an intensity of {001} orientation toan intensity of {111} orientation in the ferrite is 0.60 or more andless than 2.00 is included.
 13. The steel sheet according to claim 2,wherein the chemical composition includes, by mass %, one or more of: B:0.0001% to 0.0050%; Mo: 0.001% to 0.40%; Ti: 0.001% to 0.10%; Nb: 0.001%to 0.10%; Cr: 0.001% to 0.55%; and Ni: 0.001% to 0.25%.
 14. The steelsheet according to claim 2, wherein the secondary phase in the surfacelayer region includes one or more of martensite, bainite, and temperedmartensite.